Organ-specific metastasis is the result of a combination of specific properties of the tumor cell (the seed) and the target organ microenvironment (the soil). There is considerable evidence that one of the specific factors that influence breast metastasis is the expression of members of the matrix metalloproteinase (MMP) family. Stromal MMP activity can significantly influence the development of metastatic lesions in the lung and the bone in the MMTV-polyoma middle T model of breast cancer. Interestingly, there is site-specificity in the response. For example, stromal MMP2 facilitates the growth of breast cells in the bone but not the lung, and stromal MMP9 facilitates the growth of breast cancer cells in the lung but not the bone. In addition, there are significant genetic modifiers of the response; i.e. MMP9 effects on breast-to-lung metastasis are observed in the C57Bl/6 genetic background but not the FVB background. These organ- and strain-specific differential effects of MMPs on metastatic efficiency will be used to uncover cellular or molecular events that are critical to the development of site-specific breast metastases. In addition to a focus on metastasis to the lung and the bone, this application seeks to understand breast-to-liver metastasis, a very common site of breast metastasis yet an area that is understudied. Experiments will test the hypothesis that site-specific metastasis of breast cancer is controlled by stromal MMP activity through the regulation of rate-limiting steps in the establishment of a pre-metastatic niche, tumor cell survival and initial growth, and/or sustained growth and development of macrometastatic lesions. The aims will be to determine the contribution of stromal MMP activity to 1) the homing, attachment, survival, and initial growth, 2) the sustained growth and development of clinically detectable metastatic lesions and 3) the development of a premetastatic niche in the lung, liver, and bone microenvironments. The studies will be facilitated by state-of-the-art in vivo, ex vivo, and fixed tissue imaging capabilities to observe the steps of metastasis in real time in the target organ. In addition, they are facilitated by the previous development of novel proteolytic beacons, nanoparticle-based optical imaging agents that permit quantitative assessment of the selective activity of specific MMP family members. The results of these studies will determine the rate-limiting steps in breast cancer organ-specific metastasis that are influenced by MMP2 and MMP9. Thus, they provide basic information critical to the understanding of the molecular mechanisms underlying the effects of the soil in determining organ-specific metastasis, as well as provide guidance for future applications of selective MMP inhibitors for the prevention and/or treatment of metastatic disease.